Ajax request to the same PHP page gives no errors but doesn't work

I am making a product displaying page of an ecommerce website. The products are to be filtered by brands on the basis of brands that are checked by the customer. For this I have used an ajax request everytime a brand is checked. The problem is that the page cannot receive the get variables that i am sending to the same page. The ajax request is not giving any errors and also the chrome debugger side is also not showing any error at all. This is the page:snapshot of the products page

I am using alert() in the beforeSend in the ajax request and it returns the correct data as expected. But the PHP section of the page does not received the values. There are no errors on the PHP side or the browser debug window.